This invention relates to xerographic reproduction and specifically to transparencies which are suitable for use in a xerographic reproduction process.
Transparencies are a highly useful product in visual education since an image on a transparency may be projected with the necessary degree of magnification onto a screen where it may be viewed by a large number of persons. Transparencies have heretofore been made by photographic reproduction of the desired image and accordingly have required the skill of an individual who is familiar with complex photographic reproduction processes. Photographic reproduction of transparencies also requires the expenditure of a large amount of time and money and is thus undesirable for this reason. Therefore, an easy and inexpensive means for the production of transparencies has been sought whereby transparencies could be conveniently and economically imaged and then used an unlimited number of times in visual education programs.
The advent of xerography and electrostatic copying as generally disclosed by Carlson in U.S. Pat. No. 2,297,691 has proven to be a highly successful process for reproduction with the inherent advantages of speed and reliability. In a usual xerographic process, an electrostatic image of an object as formed on a recording member such as a xerographic plate or drum. The xerographic plate may comprise a layer of photoconductive material, such as selenium on a conductive metal backing. The latent electrostatic image which is formed on the photoconductive material is developed into a powder image which is then subsequently transferred to a sheet of paper and affixed thereon to form a permanent print.
The xerographic process has therefore proven to be an easy and reliable means for the production of transparencies. Transparencies made by a xerographic process are produced by forming an electrostatic image of the desired object, developing it, and then transferring it to a transparent sheet material with the image being permanently affixed or fused thereto by either the application of heat or by the action of a solvent vapor. In either case the toner which is used to develop the powdered image is coalesced on the sheet material by the fusing technique to form a permanent image thereon. Solvent fusion techniques, for transparent materials made by a xerographic process, for example, are illustrated in U.S. Pat. Nos. 3,049,810 and 3,148,078.
While the xerographic reproduction process is an apparent solution to the problem of economical and efficient production of transparencies, other problems have also been encountered with its use in the production of transparencies. One of the most pronounced problems with producing transparencies by an electrostatic copying process is to get the powdered or developed image to adhere well to a transparent film material before the image is permanently affixed thereto by fusing. Failure to achieve this results in partial or "cracked" images. A further problem encountered is obtaining proper and uniform density of the image after fixing or fusion and without resultant damage to the transparent film material either in the fusion process or in the transfer system employed within the machine. For this reason, various coating and combinations thereof with various types of transparent sheet materials have been previously proposed to obviate some of these difficulties. Included are various single component polymeric coatings such as are exemplified in U.S. Pat. Nos. 3,539,340; 3,539,341 and 3,535,112.
The above coatings, while of some assistance in improving adhesion of the developed electrostatic image to a transparent film material, nevertheless, are not entirely suitable when transparencies are produced by a multi-colored xerographic imaging process. The difficulties encountered with a multi-colored imaging process and transparencies produced thereby are due in part to the multi-component pigment developers required in the multi-colored imaging process and their varying degree of attraction for the transparent sheet material. Furthermore, the problem of getting the correct degree of coalescence of the toner particles in the permanent image is an even more critical matter with multi-colored imaging than with single color image development. This increased criticality is due to the fact that single color transparent images only require complete opaqueness or non-opaqueness of varying degrees to produce a transparency which has images suitable for projection.
Multi-colored transparency images, on the other hand, must allow for a certain degree of color density for each color or color combination in the image which is sufficient and uniform enough to allow projection of a uniform and true color. This, therefore, requires a different and unique combination of materials being employed and more critical controls being imposed upon the transparent materials which are used in a multi-color xerographic imaging process to obtain the correct degree of attraction of toner to the transparent sheet as well as proper coalescence of toner particles for good color density.
The instant invention relates to a transparency which fulfills all the requirements for use in the production of multi-colored electrophotographic images. The subject invention is particularly suited to overcome difficulties associated with the transfer step in the xerographic process. Many factors influence the quality of the transferred image, the most significant factors being those which effect the uniformity with which the toner powder image is transferred from the photoconductive surface to the sheet of support material. heretofore, in the process of transferring multi-layered toner powder images, as exemplified by a multi-color electrophotographic printing machine, the transfer of line copy has produced various problems. In particular, when a biased transfer roll is utilized to transfer successive toner powder images in superimposed registration to a sheet of support material, hollow characters frequently occur. Hollow characters may be defined as a toner area wherein substantially only the periphery thereof is transferred while the central portion remains devoid of toner particles. The problem of hollow characters is most pronounced for line copy reproductions; however, hollow characters frequently occur in solid area copy as well.
One approach to preparing multi-colored transparencies has been offered in copending applications Ser. Nos. 194,547 (filed on Nov. 1, 1971) now abandoned, 236,585 (filed on Mar. 21, 1972) now abandoned, and 321,511 (filed on Jan. 8, 1973) issued as U.S. Pat. No. 3,854,942. In these applications, a transparency comprising a thermoplastic film having a coating including a resin material, a wetting agent, and a particulate material is disclosed. While these transparencies function very effectively in most electrophotographic color copy machines, the nature of transfer subsystems in other color machines results in a color reproduction containing the above mentioned and undesirable hollow characters.
It is an object of the present invention, therefore, to provide for a transparency upon which a multi-colored xerographic image may be permanently affixed with uniform and consistent color reproduction and density.
It is also an object of the present invention to provide a transparency which is permanent in nature and having the sheet strength necessary to allow repeated use thereof for visual education purposes.
It is also an object of the present invention to provide for production of a color transparency by a xerographic multi-color imaging process which in turn eliminates the skill of a technician who is trained in reproduction photographic processing.
These and other objects, as well as the scope, nature and utilization of this invention will be apparent by the following detailed description and appended claims.